Timescales glass, transition

Fig. 19. Experimental spin alignment decay curves of chain deuterated PS-d3 at temperatures above and below the glass transition for various evolution times t,. Note the different timescales of t2 at the different temperatures. The straight lines indicate the decays of the plateau values on the timescale of the spin-lattice relaxation time T,. Sample characterization Mw = 141000, Mw/Mn = 1.13, atactic...

One characteristic of shear banded flow is the presence of fluctuations in the flow field. Such fluctuations also occur in some glassy colloidal materials at colloid volume fractions close to the glass transition. One such system is the soft gel formed by crowded monodisperse multiarm (122) star 1,4-polybutadienes in decane. Using NMR velocimetry Holmes et al. [23] found evidence for fluctuations in the flow behavior across the gap of a wide gap concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The timescale of these fluctuations appeared to be rapid (with respect to the measurement time per shear rate in the flow curve), in the order of tens to hundreds of milliseconds. As a result, the velocity distributions, measured at different points across the cell, exhibited bimodal behavior, as apparent in Figure 2.8.13. These workers interpreted their data... 

Most spectroscopic techniques (e.g. infrared and Raman spectroscopy) provide a snapshot view of the structure of a liquid because the timescale of the techniques is of the order of lattice vibration. However, NMR can probe much lower frequency motions, motions which are important in the glass transition and the viscosity of a silicate liquid. In addition, the timescale of the NMR experiment may be varied (by changing the magnetic field, or the type of experiment, T or T fJ, or observing quadrupolar effects) from a few hertz to several hundred megahertz. 

Fig. 7.1.4 Mechanical relaxation strength H as a function of the mechanical relaxation time r for SBR. Filled circles unaged material. Open circles material aged in air at 180 °C for 24 h. The timescale is shifted towards shorter times, because the curves are referenced to 25 °C and not to the glass transition temperature. Adapted from [Fiill] with permission from Hiithig Gmbh.

A simple model of an elastomer network is depicted in Fig. 7.1.8. The segmental motion of inter-cross-link chains is fast but anisotropic at temperatures of 100-150 K above the glass transition temperature The end-to-end vector R of such a chain reorients on a much slower timescale because it appears fixed between seemingly static cross-link points. As a result of the fast but anisotropic motion, the dipolar interaction between spins along the cross-link chains is not averaged to zero, and a residual dipolar coupling remains [Cohl, Gotl, Litl]. 

Figure 2.5 shows also the concentration dependence of the inverse Kauzmann temperature T (entropy catastrophe temperature). For the pure metal, T is much higher than the temperature T0 as discussed. The 77-line should also decrease with increasing concentration and end in the triple point(C, 7 )[2.21] as follows from its definition (AS = 0). It is interesting to note that at this point the real Kauzmann temperature and the inverse Kauzmann temperature meet. But in real systems, the amorphous phase has an excess entropy (small fraction of the entropy of fusion) when compared to the corresponding crystal, the exact amount determined from the kinetics and timescale of the glass transformation. Therefore, another glass transition temperature line with finite excess entropy must be considered, which will be parallel to the Tg-line (above it) and cross the T0- and 77-lines not exactly in the triple point. 

SSAR is observed when the binary diffusion couples listed in Table 2.4 are heated to an appropriate reaction temperature, TR. Examples of typical values of JR are given in Table 2.4. It is well known that amorphous metallic alloys tend to crystallize in laboratory timescales upon heating to temperatures close to their glass-transition temperature, T% [2.16]. For a typical practical timescale (e.g., minutes), one can define crystallization temperature as the temperature at which a significant fraction of an amorphous sample undergoes crystallization in the specified time. The time required for an amorphous phase to crystallize can be identified with t 2 of Fig. 2.6 (see discussion in Sect 2.1.3). In the low temperature regime (well below Tg), atomic diffusion in amorphous alloys is... 

Compared to natively folded proteins, compact denatured states ( MGs ) experience a modest increase in the number of water molecules in the hydration layer, and a slightly smaller perturbation of hydration water dynamics. Soluble protein-water dynamical coupling has been elucidated by simultaneous examination of transitions in protein and water dynamics as a function of temperature. Hydrated proteins at room temperature exhibit liquid-like motion on the subnanosecond timescale and behave like glasses at low temperature. The dynamical (or glass) transition between the low-temperature glassy state and room-temperature liquid-like state plays an important role in energy flow processes in proteins (see Ref [86] and Chapters 7 and 11). 

Polymeric solids exhibit a wide spectrum of local molecular motion. In the classical subdivision into thermosets, rubbers and thermoplastics, both the thermosets and the thermoplastics below the glass transition temperature would exhibit transverse relaxation time on the sub-millisecond timescale, the so-called solid regime of NMR. It is these materials which require specialised solid state imaging methods rather than the elastomeric materials where... 

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